Vitrification of mature mouse oocytes: Improved results following addition of polyethylene glycol to a dimethyl sulfoxide solution

Oocytes have been successfully cryopreserved using rapid and slow freezing procedures; however, variability in the success of replicates has limited its practical application. We have evaluated the potentially beneficial effects of adding 1 mg/ml of the polymer polyethylene glycol (PEG) (M-r 8000) to a 6 M dimethyl sulfoxide (Me2SO) vitrification solution. Stepwise addition of cryoprotectant, either with or without PEG, was performed at room temperature (19-21 degrees C). Oocytes were then loaded in plastic insemination straws and held in liquid nitrogen vapour at -140 degrees C for 3 min prior to storage in liquid nitrogen. Oocytes were warmed rapidly to room temperature and removal of cryoprotective agent was effected in the presence of 1 M sucrose solution. Viability was assessed by in vitro fertilization. Oocytes cryopreserved after exposure to 6 M Me2SO in the absence of PEG showed 60% normality, 80% fertilization, and 55% development to blastocyst, median of 11 replicate experiments (191 oocytes). Individual replicates yielded highly variable survival which ranged from 0 to 100%. The addition of PEG significantly improved oocyte normality to 95% (range, 76-100%; median of 9 replicate experiments, 301 oocytes). Rates of fertilization (91%; 60-100%) and development to blastocyst (73%; 67-92%) were also improved. The addition of 1 mg/ml PEG to a 6 M Me2SO solution resulted in greatly improved viability of oocytes following cryopreservation and vastly reduced the variability seen with the Me2SO solution alone. (C) 1997 Academic Press.